FIGURES IN THIS ARTICLE

Introduction

A remarkable part of entire power loss in an internal combustion engine is due to the undesirable friction between rubbing surfaces of mechanical components. The development of low-friction engines is necessary for the limitations in fossil fuels and tightening emission regulations. Lubricating oil has played a critical role in reducing the frictional losses. Using high viscosity oil in engines will reduce the components wear and friction but may increase fuel consumption, otherwise the thin lubricant film will decrease fuel consumption but there is a danger of higher component wear rate. Engineers and scholars are working constantly to develop and test new lubricants to meet these challenges.

Nanofluids are an innovative new class of fluids which can be engineered by suspended nanosized particles (1–100 nm) in conventional base fluids [1]. A large number of papers have reported that surface modified nanoparticles stably dispersed in lubricants are effective in enhancing load-carrying capacity, antiwear, and friction reduction properties. For example, SiO2 [2], Ni [3], zinc borate [2] nanoparticles dispersed in lubricants exhibit excellent antiwear behavior and friction reduction properties. Tao et al. [4] found that 1% is considered the best concentration for the nanodiamond particles in paraffin oil. Choi et al. [5] reported that the mixed nanofluids containing graphite and Ag nanoparticles not only showed enhanced load-carrying and antiwear properties in the FZG gear rig test but also reduced the electric-power consumption by more than 3% compared to the base oil.

According to theories in the literature, colloidal effect, rolling effect, protective film, and the third body maybe the main mechanisms of friction-reduction and antiwear of nanoparticles in lubricant [3]. The rolling friction is a dominant mechanism under low-load condition. In addition, under high-load conditions, due to the adhesion of nanoparticles the lubricate film at the asperity crests can decrease the straight asperity contact, and thus increase wear resistance. With load continuously increasing, the nanoparticles penetrate into the contact surface, preventing the rubbing surfaces from contact directly and increase the load-carrying capacity [6–8].

Besides the excellent tribological properties, most of the nanofluids studies have reported or assumed that nanofluids offer intriguing heat transfer enhancement compared to conventional fluids. Eastman et al. [1], Mintsa et al. [9], Moosavi et al. [10], Peyghambarzadeh et al. [11], and Leong et al. [12] observed that nanofluids exhibit great enhancement of thermal conductivity than conventional fluids. In order to investigate the influence of nanofluids effect on engine, bench tests were used to evaluate lubricants using a wide range of working conditions for initial screening [13]. Sarma et al. [14] observed that with the introduction of nano Cu and TiO2 into the lubricant, the performance of the engine become well than the one without nanoparticles.

In the previous studies, the single property of nanofluids such as tribology or thermal conductivity has paid more attention. In this paper, considering the influence of tribology and thermal conductivity, we carried out a comprehensive experimental study and analysis of lubricants dispersed with nanodiamond particles on friction machines and a real diesel engine. First, viscosity tests and thermal conductivity tests were conducted using Brookfield DV-II + Pro viscometer and thermal conductivity meter. Then, the friction and wear tests were performed to evaluate the tribological properties of lubricating oils with nanodiamond particles at different mass concentrations. Obviously, the engine performance is strongly impacted by lubricants quality. Thus, nanodiamond lubricants with different mass concentration would be used in AVL diesel engine bench test supported by DEUTZ (Dalian) Engine Corporation. The engine bench tests were carried out to evaluate the quality of lubricants under a wide range of realistic engine operating conditions. Reverse dragging process tests and mapping characteristics tests were brought in the bench test. In addition, SEM and TEM were also used to measure and analyze these properties in this study.

Experiments and Discussions

Preparation of Nanodiamond Lubricant.

The nanodiamond particles were prepared by Department of Engineering Mechanics (Dalian University of Technology) through the detonation of explosives in a steel container within a 80 dm3 volume. The compositions of the explosive were trotyl and cyclotrimethylenetrinitramine. The detonation soot collected was a kind of black powder. Nanodiamond carbon and other impurities were removed after treatment with oxidizing acids, thus the nanodiamond particles (the surface functionality is 9.85 Jm−2, the particle size is 50 nm, the particle density is 3.50 g/cm3) in the form of a loose and gray powder was obtained. In our tests, oil with no nanoparticles is regarded as the base oil. The base oil is high quality engine oil supplied by DEUTZ (Dalian) Engine Corporation which completely reach and transcend the API CF-4 level standard. The viscosity grade satisfying SAE 30 level standard, minimum of high temperature high shear viscosity is 3.4 mPa·s. The dispersion problem could be solved by physical and chemical modifications [15,16]. Oleic acid and laurate salts, surfactants containing hydrophilic hydroxyl and organophilic alkyl had added to the lubricants in order to enhance the dispersibility of nanoparticles in oil. Meanwhile, the nanosuspensions were prepared using an ultrasonic wave piezoelectric vibrator, generating ultrasonic pulses of 80 W at 20 KHz. It was used to sonicate the nanosuspensions continuously for 30 min in order to break down the agglomeration of nanodiamond particles. By using the above method, diamond nanofluids with mass fractions of 0.01–1% were prepared. The TEM image was employed to observe the morphology of nanodiamond particles which was shown in Fig. 1. The morphology of the monodispersed particles is spherical and the size of the single particles was found to be close to 50 nm. Figure 2 shows a good long-term stability of nanodiamond particles dispersion.

Viscosity of Nanofluids.

The Brookfield DV II+ Pro programmable viscometer (Brookfield Engineering Laboratories, Inc., Middleboro, MA), a rotation-type viscometer which uses a weight-driven rotating paddle to sense the viscosity of fluid spinning at a constant angular velocity [17], was employed to measure the viscosity of the nanodiamond lubricants with different mass fraction of 0.01%, 0.1%, 0.5%, and 1%. All viscosity measurements were recorded under the same steady state conditions, and the test temperature ranged from 25 °C to 140 °C.

The dynamic viscosity [18] is not only dependent on mass fraction of nanoparticle but also highly dependent on other parameters such as particle shape, size, mixture combinations and slip mechanisms, surfactant, etc. The results are presented in Fig. 3. Figure 3(a) shows the viscosity with the temperature ranged from 25 °C to 75 °C and Fig. 3(b) shows the viscosity with the temperature ranged from 75 °C to 140 °C. Both nanofluids and base oil, the viscosity has the same trend, decrease with the increase in temperature, and increase with the increase in concentration of nanoparticles. Compared with the base oil, viscosity value of nanodiamond lubricants achieves at 209.44 mPa·s, 211.48 mPa·s at mass concentration of 0.01% and 0.1%, decreased by 5.00%, 4.07% separately at 25 °C. While it achieves at 237.71 mPa·s, 253.37 mPa·s at concentration of 0.5%, 1%, increased by 7.8%, 14.9% separately. The base oil has a high viscosity grade than pure oil. The viscosity decrease might be assumed that for low mass concentration of nanofluids, surfactants and mixture combinations, such as oleic acid and ethanol, may play dominant roles, which dilute the nanosuspensions. And with concentration increasing rapidly, the viscosity of nanodiamond lubricants depends strongly on the mass concentration of nanodiamond particles. High viscosity may require higher oil pumping capacity, affecting engine power loss and total efficiency. High quality multilevel synthetic engine oil should have excellent high temperature wear resistance and smooth flow of low temperature startup performance. Excellent low temperature starting capacity and low resistance could pull the engine turn faster. Thus, nanodiamond lubricants with mass fraction of 0.5%, 1% would be excluded in the next tests.

Load-Carrying Capacity.

The load-carrying capacity was measured in MRS-10G four-ball friction and wear tester which has one ball above and three balls below. The diameter of the test balls are 12.7 mm, the material is Gcr15 steel. Rockwell hardness meets HRC 64–66. According to the standard GB/3142-82, the upper sample was driven at speed of 1450 rpm, the load was 770 N, maintaining the temperature of 75 °C, and the test period was 10 s. After the tests, wear scar of the steel balls were obtained by microscopes (Fig. 4). According to Table 1, the average wear scar diameter of the steel balls can be reduced by 44.6% (0.1 wt.%), 40.0% (0.05 wt.%), indicates that the addition of a certain amount nanodiamond particles can increase the wear resistance. This might be attributed to with extreme load, nanodiamond particles penetrate into the friction contacts surface, which can form a boundary lubrication film, preventing the friction surfaces from a direct contact, and thus improve the load-carrying capacity. With extremely low concentration such as 0.01 wt.%, there has little influence on load-carrying capacity. Thus, diamond lubricants with mass fraction of 0.01% would be excluded in the next tests.

Friction-Reduction Property.

The friction coefficient was measured in MMW-1A four-ball friction and wear tester under 392 N at 600 rpm and last for 60 min according to the standard SH/T0189. Keeping the test temperature at 130 °C, the friction coefficient date was collected by the compute every 10 s and the test period was 3600 s. The results are shown in Fig. 5. It shows the friction coefficients of base oil with different mass concentration of nanodiamond particles. Obviously the friction coefficients of lubricant containing nanodiamond particles are lower than the base oil. Dates presented in Fig. 5, except for 0.1%, a more sharply wave has happened in the test beginning, about the initial 10 s. Fracture of the oil film caused excessive wear at the higher temperature. For the plot of 0.1% concentration, there existed an adaption period in the early stage of friction test, and then it became stable, which has been remained at this level. The average friction coefficient of the diamond nanofluids at the concentration of 0.05 wt.%, 0.08 wt.%, and 0.1 wt.% is decreased by 12.5%, 15.1%, and 30.3% separately, as compared to the base oil without nanodiamond particles.

The SEM images of worn surfaces by using nanodiamond particles (0.1 wt.%) added into base oil have been shown in Fig. 6. In the base oil without nanodiamond particles, the worn surface Fig. 6(a) was destroyed which may be caused by the directly contact. While the worn surface Fig. 6(b) which using nanodiamond particles as additives was smoother than the base oil. The results are in accordance with the friction reduction behavior of nanodiamond particles.

Thermal Conductivity Tests.

In thermal conductivity measurement, a vessel containing the test sample was placed in a constant temperature-controlled oil bath. The transient short hot-wire technique was applied to measure the thermal conductivities of the diamond nanofluids from 60 °C to 120 °C. All thermal conductivities were recorded until the sample kept at the testing temperature for 10 min to ensure thermal equilibrium. Every sample was tested for 10 times at each temperature and then took the average. Figure 7 depicts the thermal conductivities of base oil and nanodiamond lubricants with mass fractions of 0.05%, 0.08%, and 0.1%. A slight increase in thermal conductivities can be found for the measured samples. The average thermal conductivity of the nanodiamond lubricants have been increased by 0.4%, 1.1%, and 1.5%, as compared to the base oil without nanodiamond particles. The improvements in thermal conductivity were not obviously for low concentrations of nanoparticles.

Engine Bench Tests.

The bench tests have been performed on AVL diesel engine bench test system supported by DEUTZ (Dalian) Engine Corporation to evaluate the variation of engine performances with different mass concentration of nanodiamond lubricants. The main technical specifications of the bench test system are listed in Table 2 and the controlled test conditions are listed in Table 3. Figure 8 shows the AVL diesel engine bench test system.

Reverse Dragging Process Tests.

The electric dynamometer and engine speed were tested continuously in the process of reverse dragging on the diesel engine. This method can be used to evaluate the entire engine's mechanical loss. It was the summation of the losses arising from the functioning of many components of the engine. Research shows that the friction loss accounts for about 50–60% of the total loss [19]. And piston assembly friction loss accounts for the largest share of the mechanical losses in the engine. It is crucial to decrease the friction loss using high quality lubricants. In this paper, the total mechanical power loss was measured with motoring test method by AVL electric dynamometer under different working conditions. Figure 9 shows that the applications of nanodiamond lubricants make the torques and powers of diesel engine have a slight decrease during the reverse dragging process. Compared to base oil, the torque has decreased by 1.7%, 1.9%, and 2.5% with nanoparticles mass friction of 0.05%, 0.08%, and 0.10% when the engine at the rated speed (2300 rpm). In the meantime, the power decreased by 0.39%, 0.41%, and 1.21%. This shows that adding nanodiamond particles to engine oil can reduce the mechanical loss which in turn conveys a good antifriction performance. The collected dates were total friction torque and power during processes of reverse dragging in our tests. However, the diamond nanofluids were just used to lubricate the piston assembly. It may be the main reason for the slight increment.

Mapping Characteristics Tests.

The economic efficiency in operation of diesel engine is an important performance of the engine. Economic efficiency in diesel engine utilization refers to the performance that keeps lowest cost to achieve efficient transportation. It is a composite target to evaluate economic efficiency of the engine operation. One common way to reveal the operating characteristics of a diesel engine over its full load and speed range is to plot fuel consumption contours on a photograph of effective engine powers (or pressures) versus engine speeds. According to statistic information, engine mapping characteristics curves are drawn by using the fitting tools and statistical software package of origin. Thus, the improvements in the performance over the engine's entire working regions are displayed in Fig. 10. The X-coordinate shows the speed of diesel engine, the Y-coordinate shows the torque, the Z-coordinate shows the power. The closed isograms represent the “Brake Specific Fuel Consumption” (g/kW·h). The fuel consumption isometrical curves are also drawn in the pictures.

According to the mapping characteristics curves we can see, the best economic zone of base oil is located in the region of low speeds and low loads, the shape is irregular and the area is small. While the best economic zone of nanodiamond lubricant is located in the central region, it has regular shape and wider area, indicating that the fuel consumption changed little in variable speeds and loads. Especially the 0.1 wt.% one, it has the largest area of best economic zone. It is obviously that the fuel economy in diesel engine has a great improvement with the application of nanodiamond lubricants.

Conclusions

To understand in detail the effect of nanodiamond lubricants on the performance of a diesel engine, tests were carried out on friction machines and the AVL diesel engine bench under realistic engine operating conditions. Additionally, investigations were conducted using viscometer, thermal conductivity meter, SEM, and TEM to interpret the possible influence mechanisms of tribology and thermal conduction with nanodiamond particles. The following conclusions were drawn from this work.

The addition of nanodiamond particles to engine oil significantly improved load-carrying capability and also substantially reduced friction coefficient. Both nanofluids and base oil, the viscosity has the same trend, decreased with increasing in temperature, and increased with the concentration of nanoparticles increasing. At low mass concentrations, nanofluids consisting of nanodiamond particles have been shown to enhance the thermal conductivity slightly. The heat transfer coefficient increased with the concentration increasing. The AVL diesel bench test results indicate that adding nanodiamond particles to engine oil has a perceptible effect on engine performance, increasing the maximum engine power and the maximum torque, reducing entire engine's mechanical loss, improving fuel efficiency, and enhancing the heat transfer. Addition of nanodiamond particles in the concentration range 0.05–0.10% in engine oil decreased engine friction by 0.18–0.27% in motored tests.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (Grant Nos. 51276031, 51376002, and 51476019). The authors gratefully appreciate the support of DEUTZ (Dalian) Engine Corporation for the bench tests reported in this paper.

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